This invention relates generally to electric motors, and more particularly to a brushless D-C motor which is controlled by electronic switches responsive to galvanomagnetic transducers.
A known brushless D-C motor which uses a pair of Hall-effect generators as galvanomagnetic transducers to control rotation is described in "Electrical Equipment", 1972, No. 1, pp 21-23, at FIG. 1. The brushless D-C motor is described therein as a 180.degree. circuit, having two winding legs which are disposed 90.degree. apart. Each of the windings carries an electric current responsive to a series-connected transducer for controlling rotation of a motor rotor throughout a rotation angle range of 180.degree.. A transistor pair which is associated with windings disposed 180.degree. apart is controlled by control signals which are produced by one of two Hall-effect generators which are disposed 90.degree. with respect to one another. The Hall-effect generators produce a sinusoidal output signal which corresponds to the angle of rotation of the rotor, which signals control transistors which are biased in the linear region. Thus, the currents which flow through the individual windings in response to the control by such transistors have a waveform which corresponds approximately to a sinusoidal half-wave.
In theory, the 180.degree. circuit requires few electronic components and should have negligible torque ripple. Such small ripple results from the mathematical relationship: EQU D.about.I.sub.Wn .multidot. sin.sup.2 .gamma.+I.sub.Wn+2 .multidot. cos.sup.2 .gamma.
In this expression, .gamma. represents the angle of rotor rotation; I.sub.Wn and I.sub.Wn+2 represent the currents flowing through respective, spacially successive windings; and D is proportional to the torque. It can be seen from this expression that if windings currents I.sub.Wn and I.sub.Wn+2 are equal, the torque is proportional to the squares of the trigonometric functions, which is equal to 1. Thus, torque is not a function of .gamma., thereby producing zero torque ripple. However, such zero torque ripple is not obtainable in practice because the transistors which control the current through the winding have different cutoff values and current gains, and the Hall-effect generators have different sensitivities and threshold voltages. Thus, practical applications of the known 180.degree. circuit produce torque ripples of up to 50%.
It is, therefore, an object of this invention to provide a brushless D-C motor having reduced torque ripple.